This invention relates to a toner for developing electrostatic images in an image forming method such as electrophotography, electrostatic printing or electrostatic recording.
In a dry copying method, an electrostatic latent image on a photosensitive medium is developed with a toner composed of a binder and a coloring agent. The developed toner image is transferred to a transfer member such as paper and fixed there. Residual toner which remains untransferred from the photosensitive medium to paper is removed with a cleaning device. From the standpoint of economy, maintenance and waste treatment, however, it is undesirable to produce waste toner.
JP-A-6-51672 discloses an image forming device including a rotatable photosensitive drum around which there are provided a developing zone, transfer zone and a cleaning zone. The cleaning zone has a cleaning roller electrically coupled with means for applying a voltage of a selected polarity. When that portion of the photosensitive drum which carries residual toner and which is fed to the cleaning zone is to be used for contacting with paper in the next transferring step, the voltage applying means is operated to apply a voltage of a polarity opposite to that of the residual toner, so that the residual toner is removed by the cleaning roller. On the other hand, when that portion of the photosensitive drum fed to the cleaning zone is not to be used for contacting with paper in the next transferring step, the voltage applying means is operated to apply a voltage of the same polarity as that of the residual toner, so that the toner removed from the photosensitive drum is again deposited to that portion of the drum. The toner thus returned to the drum is collected in the developing zone.
With this device waste toner is not produced. It has been found, however, that the transfer of the toner between the cleaning roller and the photosensitive drum is not able to be smoothly carried out. As a result, the background of copied images is apt to be stained.